Hi @TheMapSmith,
The current Docker install we have is meant to be a preview - it's not meant for production instances since it’s using a known password string and contains other non-production configuration settings.
Upgrading will erase the data in the docker container, which is likely the issue you ran into. We're thinking about updating it to work for production, as several community members have asked for it.
There is also a production docker image built by the community you can try out, but it hasn't been tested by the Mattermost team.